Thermoplastic elastomers (TPEs) represent an important segment of the worldwide elastomer market due to their combination of mechanical properties comparable to vulcanized rubbers and straightforward processing similar to that of thermoplastics. See Legge et al., Thermoplastic Elastomers, Munich: Hanser, 1987. TPEs are also of interest because of their capacity to self-assemble to form ordered phase separated structures having nanoscale dimensions, with morphologies and properties tunable by controlling the volume fractions of hard and soft segments. See Rader, C. P., in Modern Plastics Encyclopedia, Volume 72, New York: McGraw-Hill, 1996.
Linear ABA triblock copolymers, including styrenic block copolymers (SBC), are often usable as TPEs. The SBCs include, for example, the SIS and SBS triblock copolymers, where S represents glassy polystyrene (PS) end-blocks and I and B represent rubbery polyisoprene (PI) and polybutadiene (PBD) mid-blocks, respectively. Some of these copolymers are sold under the trade name KRATON™ (Kraton Polymers, Houston, Tex., United States of America). For these block copolymers, the morphology formed, and thus the mechanical properties, are directly linked to the volume fractions of the two components.
Star block copolymers, e.g., star polymers where each arm is a block copolymer, having PS outer blocks and PI inner blocks, have been reported as having improved tensile strength relative to linear triblocks of comparable composition and segment molecular weights. See Bi and Fetters, Macromolecules, 9, 732-742 (1976). Commercial star block copolymers based on S/I and S/B compositions have been commercialized by companies such as the Phillips Petroleum Company (now part of ConocoPhillips Company, Houston, Tex., United States of America), and BASF (Ludwigshafen, Germany).
Graft copolymers comprising PI backbones and regularly spaced PS side chains attached to the backbone at branch points have also been shown to have elastomeric properties. See, e.g., Uhrig and Mays, Macromolecules, 35, 7182-7190 (2002); and Mays et al., Macromol. Symp., 215, 111-126 (2004). However, the synthesis of regular spaced copolymers can be challenging. See Uhrig and Mays, Macromolecules, 35, 7182-7190 (2002).
Accordingly, there is need in the art for additional elastomeric polymers and materials that can be produced therefrom, that can be synthesized easily and at low cost. There is also a continuing need for elastomeric polymers that have mechanical properties that can be readily fine-tuned particular end uses.